1. Field of the Invention
The present invention relates to a method and device for forming an image, and more particularly to a method and device that can effectively adjust an imaging position of an optical beam spot formed on a scanned surface.
2. Discussion of the Background
An optical scanning device is commonly known in relation to an image forming apparatus such as a digital copier, a laser printer, a facsimile, an optical plate-maker, an optical plotter, and other similar devices. Recently, recording density in digital copiers and laser printers has been increasing, and an optical beam spot having a smaller diameter is formed on a surface of a photoconductive medium. In addition, it has become common to use a resin lens in a scanning imaging optical system (i.e., an optical system in which a deflected light flux forms an image on the surface of the photoconductive medium) from view point of cost and ease of forming a lens surface into a special shape.
In order to form an optical beam spot having an appropriate diameter on the surface of the photoconductive medium, the optical beam spot needs to be precisely formed. Thus, an adjustment of an imaging position of the optical beam spot is required. The optical beam spot is precisely formed on the surface of the photoconductive medium when each optical element is accurately located as designed in an optical path formed from a light source to the photoconductive medium. However, when errors caused in processing or assembling the optical elements are considered, it may not be practical to arrange the optical elements as designed.
Degrees of variance of a radius of curvature and refractive index of a resin lens caused by change in environmental temperature is large compared to those of a glass lens. Thus, if the environmental temperature changes, a change in a shape and variance of refractive index of the resin lens are caused, and the imaging position of the optical beam is then shifted from the surface of the photoconductive medium. Hence, the diameter of the optical beam spot is increased, resulting in degradation of a produced image due to lowered resolution.
In an optical scanning device, an optical system provided between the light source and photoconductive medium generally includes an anamorphic optical system. In the anamorphic optical system, an image in the main scanning direction is differently formed from the image formed in the sub-scanning direction. Thus, the diameter of the optical beam spot is differently increased between the main and sub-scanning directions when a resin lens is used.
A method of correcting a shift of an imaging position of an optical beam spot caused by change in an environmental temperature is disclosed in Japanese Patent Laid-Open Publication No. 8-292388. In the method, the imaging position of the optical beam spot is oppositely shifted in positive and negative lenses, respectively. A resin lens having a power that is opposite to a power of a resin lens in a scanning imaging optical system is provided in an optical path formed from a light source to an optical deflector to offset the shift of the imaging position of the optical beam spot.
Another method is disclosed in Japanese Patent Publication No. 2804647: either a positive or negative resin lens, which is opposite to but has a same power as a resin lens in the scanning imaging optical system, is provided between a light source and optical deflector so as to correct a shift of the imaging position of the optical beam spot on the surface of the photoconductive medium in the main scanning direction. In addition, a location of the scanning imaging optical system is regulated to a level in which a degree of the shift of the position of the optical beam spot in the sub-scanning direction is negligible.
The present invention has been made in view of the above-mentioned and other problems, and addresses the above-discussed and other problems.
The present invention advantageously provides a novel method of adjusting an imaging position of an optical beam spot in both main and sub-scanning directions in an optical scanning device.
According to an exemplary embodiment of the present invention, the method includes an adjustment of the imaging position of the optical beam spot in the main scanning direction by adjusting a position of at least one lens having a power in the main scanning direction in an optical axis direction, and an adjustment of the imaging position of the optical beam spot in the sub-scanning direction by collectively adjusting a position of a linear image imaging optical system as a whole in the optical axis direction.
The present invention further advantageously provides an optical scanning device in which a shift of the imaging position of the optical beam spot caused by change in environmental temperature due to use of a resin imaging element is properly corrected.
Other objects, features and advantages of the invention will become apparent to those skilled in the art when reading the following Detailed Description with reference to the accompanying drawings.